"The boy is eating his apple."
Translation:Gutten spiser eplet sitt.
Here's a link to the Tips & Notes - Possessives. I don't know for certain that it'll open on your mobile:
"Si/sin/sitt/sine" is used to point the ownership back to the subject of the sentence. This distinction is not made in English, which makes for ambiguous sentences:
"Han liker boka si."
"He likes his book." (referring to his own book)
"Han liker boka hans".
"He likes his book." (referring to some other guy's book)